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Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Available-for-sale Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at the start of the period $ 6,314 $ 6,891
Revaluation (87) (365)
Settlements 0 (299)
Asset Sales 0 0
Balance at the end of the period 6,227 6,227
Silvertip Mine    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at the start of the period 48,289 47,965
Revaluation 0 0
Settlements 0 0
Liability Sales 327 651
Balance at the end of the period $ 48,616 $ 48,616